PROPERTIES
Properties of Chemical
Bonds
Why
Make a Bond?
Why should atoms bond at
all? In nature we find that some elements like He, Ne, and Ar are never found
bonded to other atoms whereas most other elements are only found bonded to
other elements. What makes the noble gases so special? The answer lies in their
closed shell electron configurations. Because the valence shell of a noble gas
is completely full, it cannot accept another electron into the shell. The
nucleus is positively charged and pulls on the electron, so the loss of an
electron from a noble gas is unfavorable. Therefore, like real nobility, the
noble gases do not want to do anything at all--that is, noble gases are
unreactive because they have filled valence shells.
Any element other than a
noble gas has an open shell configuration, which is unstable relative to the
configuration of a noble gas. Non-noble atoms react to form bonds in an attempt
to achieve a closed shell electron configuration. For example, when a lithium
atom and a fluorine atom meet, as shown in , lithium can achieve a noble gas configuration,
1s2, by donating an electron to fluorine which also achieves the
noble gas configuration 1s22s22p6:
The above reaction
represents the formation of an ionic bond. The negatively charged anion, F, and
the positively charged cation are held together in the bond by the attraction
of unlike charges as dictated by Coulomb's law. You may have asked yourself why
two fluorine atoms don't come together to perform the following reaction:
Even though the reaction
may appear to be favorable because of its production of a closed shell species,
there is a way to have both F atoms achieve a noble gas configuration. By sharing
their electrons, each fluorine atoms can have a complete octet
in its valence shell. Such a sharing of electrons is called a covalent bond and
will be discussed in depth in a separate section.
Properties
of a Bond
The way bond properties
were chosen to characterize bonds have a historical basis. Scientists made
their first rational attempts to describe bonding by looking at data they could
collect about bonds. We too will look at the experimental data on bonds to try
to analyze bonding.
Perhaps the most useful
aspect to know of a bond is its strength. Weak bonds are easily broken and
molecules with such bonds are fairly reactive. Conversely, strong bonds are
difficult to break and give rise to stable molecules. Therefore, it is sensible
to define bond strength as the amount of energy needed to break a chemical
bond. Trends in bond strength show that homoatomic bonds (those formed between
atoms of the same element) tend to be strong. But going across a row in the
periodic table, the trend in bond strength may not be regular. For example,
period 2 elements have the following strength order: Li-Li > Be-Be < B-B
< C-C < N-N > O-O > F-F. This irregular trend is repeated in period
3 homoatomic bonds. If we look at bond strength data, we also notice that the
Li-F bond is several times stronger than the F-F bond or the Li-Li bond. It is
not important for you to memorize such trends. We use them to show that
whatever theory of covalent bonding we propose must account for these
observations.
Bond lengths follow the
expected trend that bonds between larger atoms are longer and bonds between
smaller atoms are smaller. What is surprising is that bond strength and bond
length are inversely related--a short bond is generally stronger than a long
one. Another unexpected piece of bond length data shows that there are three
common bond lengths for C-C bonds. Our bonding theory must also predict the
above trends in bond length.
Bond dipole data provides
at least a partial answer for several of the above observations. Because F is a
more electronegative atom than C, the electrons in a C-F bond will be polarized
toward F. The fluorine atom then acquires a large partial negative charge (δ-)
and the carbon atom a large partial negative charge (δ+) as shown in
:
The crossed arrow
underneath the C-F bond indicates that there is a partial positive charge
located on C and a partial negative charge located on F. The charge separation
leads to a coulombic attraction between the two atoms in the bond and makes the
C-F bond stronger than either the C-C or F-F bonds, which have no bond dipoles.
This stronger bond will have the nuclei closer than the sum of their atomic
radii due to the coulombic attraction between the oppositely charged ends of
the molecule.
In the next section we will
take a closer look at ionic bonding.
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